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<title>Issue 2566: Requirements on the first template parameter of container adaptors</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++17">C++17</a> status.</em></p>
<h3 id="2566"><a href="lwg-defects.html#2566">2566</a>. Requirements on the first template parameter of container adaptors</h3>
<p><b>Section:</b> 23.6 <a href="https://wg21.link/container.adaptors">[container.adaptors]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2015-12-08 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#container.adaptors">issues</a> in [container.adaptors].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
As noted in <a href="http://stackoverflow.com/questions/34128782/what-is-the-use-of-first-template-parameter-in-priority-queue">this 
StackOverflow question</a>, 23.6 <a href="https://wg21.link/container.adaptors">[container.adaptors]</a> doesn't seem to place any requirement on the first template parameter 
(<code>T</code>) of <code>stack</code>, <code>queue</code>, and <code>priority_queue</code>: the only use of <code>T</code> is in the default template 
argument (which need not be used) for the second template parameter (<code>Container</code>), while all of the operations of the adaptors 
are defined using <code>Container</code>'s member typedefs.
<p/>
This permits confusing and arguably nonsensical types like <code>queue&lt;double, deque&lt;std::string&gt;&gt;</code> or 
<code>priority_queue&lt;std::nothrow_t, vector&lt;int&gt;&gt;</code>, which presumably wasn't intended.
</p>

<p><i>[2016-02, Issues Telecon]</i></p>

<p>
P0; move to Tentatively Ready.
</p>


<p id="res-2566"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4567.</p>

<ol>
<li><p>Edit 23.6.1 <a href="https://wg21.link/container.adaptors.general">[container.adaptors.general]</a>/2 as indicated:</p>

<blockquote>
<p>
-2- The container adaptors each take a <code>Container</code> template parameter, and each constructor takes a <code>Container</code>
reference argument. This container is copied into the <code>Container</code> member of each adaptor. If the container
takes an allocator, then a compatible allocator may be passed in to the adaptor's constructor. Otherwise,
normal copy or move construction is used for the container argument. <ins>The first template parameter <code>T</code> of the container 
adaptors shall denote the same type as <code>Container::value_type</code>.</ins>
</p>
</blockquote>
</li>
</ol>





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